Fluctuations of fragment observables

This contribution presents a review of our present theoretical as well as experimental knowledge of different fluctuation observables relevant to nuclear multifragmentation. The possible connection between the presence of a fluctuation peak and the occurrence of a phase transition or a critical phenomenon is critically analyzed. Many different phenomena can lead both to the creation and to the suppression of a fluctuation peak. In particular, the role of constraints due to conservation laws and to data sorting is shown to be essential. From the experimental point of view, a comparison of the available fragmentation data reveals that there is a good agreement between different data sets of basic fluctuation observables, if the fragmenting source is of comparable size. This compatibility suggests that the fragmentation process is largely independent of the reaction mechanism (central versus peripheral collisions, symmetric versus asymmetric systems, light ions versus heavy ion induced reactions). Configurational energy fluctuations, that may give important information on the heat capacity of the fragmenting system at the freeze out stage, are not fully compatible among different data sets and require further analysis to properly account for Coulomb effects and secondary decays. Some basic theoretical questions, concerning the interplay between the dynamics of the collision and the fragmentation process, and the cluster definition in dense and hot media, are still open and are addressed at the end of the paper. A comparison with realistic models and/or a quantitative analysis of the fluctuation properties will be needed to clarify in the next future the nature of the transition observed from compound nucleus evaporation to multi-fragment production. 1 Fluctuations and phase transitions Since the first inclusive heavy ion experiments, multifrag-mentation has been tentatively associated to a phase transition or a critical phenomenon. This expectation was triggered by the first pioneering theoretical studies of the nuclear phase diagram [1] which contains a coexistence region delimited, at each temperature below an upper critical value, by two critical points at different asymmetries[2, 3]. Even more important, the first exclusive multifragmen-tation studies have shown that multifragmentation is a threshold process occurring at a relatively well defined deposited energy[4,5,6,7]. The wide variation of possible fragment partitions naturally leads to important fluctuations of the associated partition sizes and energies. Different observables have been proposed to measure such fluctuations. Using the general definition of the n−th moment as